timing_test_solution.py

import cv2
import time
import numpy as np


IMG_SIZE = (250, 250)
NUM_ITERS = 500


def compare_tobytes_runtimes(img1, name1, img2, name2):
    t0 = time.perf_counter()
    for _ in range(NUM_ITERS):
        img1.tobytes()
    img1_t = time.perf_counter() - t0

    t0 = time.perf_counter()
    for _ in range(NUM_ITERS):
        img2.tobytes()
    img2_t = time.perf_counter() - t0

    print(f"Ratio of {name1}:{name2} = {(img1_t / img2_t):0.5f}")

if __name__ == "__main__":
        
    # Random image
    img_rand = np.random.randint(0, 256, IMG_SIZE, dtype=np.uint8)

    # Save it, turning it into a "real" image and making this script self sufficient.
    cv2.imwrite("real_image.png", img_rand)

    # Real image
    img_real = cv2.imread("real_image.png")
    img_real = img_real[:, :, 0]

    compare_tobytes_runtimes(img_real, "Real", img_rand, "Random")

    #
    # Why the near 100x difference?
    #

    # 1. Are they the same sizes?
    assert img_rand.shape == img_real.shape
    print("\nShape is the same:", img_rand.shape)

    # 2. Are they the same data types?
    assert img_rand.dtype == img_real.dtype
    print("\nDtype is the same:", img_rand.dtype)

    # 3. The actual data is different. Do different int8s takes different times to serialize?
    for x in range(img_rand.shape[0]):
        for y in range(img_rand.shape[1]):
            img_real[x, y] = img_rand[x, y]
    assert img_real.tobytes() == img_rand.tobytes()
    print("\nRerunning timing comparison after copying data from 'random image' to 'real image'...")
    compare_tobytes_runtimes(img_real, "Real", img_rand, "Random")

    #
    # Still large discrepency in runtimes
    # Reload the 'real image'
    # 
    img_real = cv2.imread("real_image.png")
    img_real = img_real[:, :, 0]

    print("\nCheck out the flags on the numpy matrices")
    print("Real image:")
    print(img_real.flags)
    print("Random image:")
    print(img_rand.flags)

    # It's a memory layout / memory access related issue.
    # Lets call copy() on the 'real image' first.
    print("\nRerunning timing comparison after calling copy() on the 'real image'...")
    img_real = img_real.copy()
    compare_tobytes_runtimes(img_real, "Real", img_rand, "Random")

    print("\nNOW THE TIMES ARE ABOUT THE SAME :)")

    print("\nThe flags also match!")
    print("Real image:")
    print(img_real.flags)
    print("Random image:")
    print(img_rand.flags)